Range winding topology activates to increase the transformer turn-ratio for hold up time when the input DC voltage drops to certain value, so the converter is only required to be designed for narrow range input and it operates with large duty cycle with easy soft switching in the primary side resulting in good efficiency. At the same time range winding topology needs small current rating components because only certain hold up time, such as 20 ms, is its operation. However, transient problems occur during range winding activation because of the nonlinearity of range winding topology performance and the limitation of the close-loop bandwidth.
Conventional secondary side post regulators (SSPR) control the pulse width to regulate the tight output voltage and the primary side switches operate at 0.5 duty cycle, resulting in very easy conditions for soft switching in the primary side. But post regulators introduce extra conduction loss in the secondary side. Also post regulators cannot reduce the voltage stress across the secondary side rectifiers in wide range input DC/DC converter because the converter is required to be designed in the lowest DC input voltage and operates in the highest DC input voltage, so the efficiency resulting from the use of a conventional post regulator is poor.
In medium power rating converters, a half bridge converter is attractive because of its simplicity and low transformer turn-ratio. In wide range input DC/DC converters, conventional half bridge does not achieve zero voltage switching (ZVS) in the primary side because of small duty cycle operation. Asymmetrical half bridge converter and duty cycle shifting (DCS) control can achieve ZVS under certain conditions, but it does not provide ZVS in full range loads. In wide range input conditions, the efficiency of the half bridge converter is low. The best efficiency of isolated converters such as half bridge, full bridge and push pull topologies, exist in the conditions of approximately 0.5 duty cycle in the primary side, there is still the problem of regulating the output voltage.
Generally, wide range input DC/DC converters are designed at the minimum input voltage Vin min and always operates at the maximum input voltage Vin max with a small duty cycle, which makes very difficult to achieve soft switching in the primary side switch while achieving higher voltage stresses in the secondary rectifier that have large conduction and switching losses, so that the resulting converter has low efficiency.
FIG. 1 is a graph showing the operational point and design points for prior art front end DC/DC converters. With a requirement for a holdup time when the line input dropout occurs, the converter has to provide the regulated output voltage during holdup time, which means front end DC/DC converter has to operate in wide range input voltage between maximum and minimum voltage as shown on the graph. Most of time the converter operates at high input Vin max, and it only operates for very short holdup time when AC line dropout. So the converter should be designed in lowest voltage Vin min and it operates in high input voltage Vin max with very small duty cycle, which makes it very difficult to achieve soft switching and high voltage stresses across the secondary side rectifiers, all of those result in low efficiency in wide range input DC/DC converter.
FIG. 2 shows an example of a prior art half bridge converter and FIGS. 3a, 3b and 3c show different types of conventional prior art secondary side post regulators. FIG. 3d shows the range winding topology. Secondary side post regulators regulate the output voltage using secondary side control and the primary side easily achieves zero-voltage-switching in full range load due to 50% duty cycle operation. Main rectifiers are composed of rectifiers D1 and D2, filter Lf and output capacitor Co and the post regulator composed of rectifiers D3, D4 and switches Q3 and Q4.
In FIG. 3a, switches Q3 and Q4 are connected in series with the main rectifiers to control the pulse width for tight regulation of the output voltage with ZVS if the switches turn on or off at the intervals of main rectifiers' current commutation, but this series type SSPR it introduces large conduction loss due to large current ratings in the current path especially in low voltage applications. At the same time, this series type SSPR cannot reduce the voltage rating across the main rectifiers in wide range input converter in wide range input converter.
In FIG. 3b, switches Q3 and Q4 are series with D1 and D2 respectively and they can achieve ZCS. However, this is a cascade system so total converter efficiency is a concern during design. Also the cascade system cannot reduce the voltage rating across the main rectifiers in a wide range input converter.
The schematic diagram of FIG. 3c shows a combination of the topology of FIG. 3b and the rectifier in FIG. 1 to reduce the output filter inductance and to reduce the voltage rating across the main rectifiers in a wide range input converter. However, the combination increases conduction losses in primary side switches and post regulator because of the high current rating in the current path when the post regulator is activated.
In FIG. 3d, switch Q3 of range winding topology turns on when primary input voltage drops to certain value, the increased transformer turn ratio extends the hold up time so that the range winding topology can be designed in narrow input range. However, the transient response is the problem because of nonlinearity of range winding topology and bandwidth limitation.